The present invention relates to a collimator for use in radiotherapy.
Radiotherapy is routinely employed for the treatment of invasive medical conditions such as cancer. The essential principle of radiotherapy is that the applied beam is apt to kill cells in its path. Thus, if the beam is directed at a cancerous or otherwise abnormal area the cells would eventually destroyed. However, it is inevitable that damage will also be caused to surrounding healthy tissue, and if this is not appropriately limited then the side effects for the patient could well be severe.
Such limitation is commonly achieved by collimating the beam so as to extend only over the minimum necessary area, to include all of the cancer but only a minimum amount of healthy surrounding tissue. Multi-leaf collimators are well known for this purpose, and comprise two banks of individually extendable leaves which can be extended into and out of the radiation beam so as to shape the emergent beam at will. FIG. 1 shows the effect of a multi-leaf collimator. The leaves 10 are extended into the beam and prevent radiation reaching the patient in those areas. Each leaf 10 is typically stopped just short of the area 12 to be irradiated. Thus, the areas beneath the leaves (such as at 14) are not irradiated, limiting side effects of the treatment.
FIG. 1 also illustrates the inherent limitation of multi-leaf collimators, which is that areas 16 are unnecessarily irradiated due to the finite width of the leaves 10. Efforts have therefore been directed in producing multi-leaf collimators with narrower leaves, to achieve higher spatial resolution and limit the size of the unnecessarily irradiated area 16. However, this presents significant engineering difficulties since the leaves must be relatively deep in order to present a high opacity to x-rays. In addition, there must be no gaps between the leaves through which radiation could leak. Thus, the engineering difficulties in accurately moving close fitting narrow heavy leaves are significant. Nevertheless, so-called xe2x80x9cmicro-multi-leaf collimatorsxe2x80x9d have been developed. These typically have a very limited field size in order to alleviate the engineering difficulties.
The present invention seeks to provide a collimator in which the irradiation pattern of such a micro multi-leaf collimator can be achieved without the associated engineering difficulties and limited field size.
It therefore provides a radiotherapy apparatus comprising a first collimator and a second collimator, the first collimator comprising a plurality of elongate leaves lying alongside each other, each being moveable longitudinally, the second collimator comprising a plurality of slits, the first and second collimators being aligned such that each slit of the second collimator is associated with at least one leaf of the first collimator, the slits having a width which corresponds to a fraction of the width of the leaves.
The slit widths correspond to the leaf widths in that the projected image of the slits on the patient is a fraction of the projected image of the leaves on the patient. Where the first and second collimators are immediately adjacent, this will mean that the physical width of the slits is a fraction of the physical width of the leaves. Where the two collimators are at different locations along the beam, an appropriate scaling will be necessary such that the projected dimensions at the patient are an appropriate fraction.
It is preferred if the first collimator is above the second, but this is not in fact necessary and the second collimator can well be above the first.
It is also preferred that the slits are focused on the radiation source.
According to the invention, a first irradiation is made, during which the first collimator will define the outer edge of the irradiation pattern, and the second collimator will serve to narrow the effective width of each leaf of the first collimator. This narrowing is a simple function of the relative widths of the slits of the second collimator and the leaves of the first. This will leave gaps in between the slits of the second collimator, which can then be filled by moving one or more of the patient, first and second collimators, so as to irradiate an area omitted in the first irradiation. In this second irradiation, the positions of the leaves of the first collimator are adjusted as necessary. This process is then repeated until the entire target area has been irradiated.
Thus, by use of the collimator the present invention allows the resolution of a micro multi-leaf collimator to be provided with no associated engineering difficulties, the only cost being the need for repeat irradiations. The treatment time for each irradiation could be reduced by selecting higher dose rates.
One way of providing the repeat irradiations is to make the second collimator movable relative to the first. However, it is preferable for the first and second collimators to be movable together relative to the patient. This could obviously be achieved either by moving the collimators en bloc, or by fixing the first and second collimators and moving the patient. Alternatively, the patient could be moved continuously relative to the source, the first collimator leaves being adjusted as necessary as the treatment progresses.
The second collimator is preferably removable from the apparatus, thereby to enable exchange with alternative second collimators. These alternative second collimators could have a different fractional width, or a different irradiatable area.
Suitable values for the fraction are {fraction (1/2, 1/3, 1/4)} or ⅕.
In general, narrow slits will result in a greater number of repeat irradiations. This means that the opacity to X-rays of the second collimator must be correspondingly greater, which implies a more massive device.
Wider slits will, in general, reduce the treatment times, but will not achieve as good conformation to the desired irradiation area. It will therefore be advantageous to choose the widest slits commensurate with the desired accuracy.
The present invention therefore also relates to a kit of parts comprising the apparatus as defined above in combination with alternative second collimators exhibiting different fractional widths.